This application is based upon, claims the benefit of priority of, and incorporates by reference, the contents of prior Japanese Patent Application No. 2002-65589 filed Mar. 11, 2002 and No. 2003-11051 filed Jan. 20, 2003.
1. Field of the Invention
The present invention relates to a control system for electric power assisted steering apparatuses that use a motor.
2. Description of the Related Art
Generally, electric power assisted steering has been commercialized to help the driver with rotation of the steering wheel by utilizing motor torque. This apparatus calculates a control current to be provided to the assist motor in accordance with the torque (output from the torque sensor) needed for rotation of the steering wheel. Thus, if the CPU in the ECU that calculates this control current provides the wrong output because of a CPU problem or the like, the motor assist falls short or exceeds the appropriate level, and steering with the steering wheel becomes more difficult.
To solve this problem, Japanese Patent Laid-Open Publication No. 2002-67988 discloses a technique for deactivating the steering assist system unless a CPU calculation result falls in a predetermined range of the torque sensor output, deciding that a calculation problem has arisen (see pages 2-4, FIGS. 4 and 5 in this publication).
Presently, to improve steering performance, a damping correction is carried out to correct the extent of motor assist based on the steering speed, for example. In this case, the control current for steering assist is calculated by adding a basic control quantity calculated in correspondence to the steering torque and a modified control quantity calculated in correspondence to the steering speed. Thus, the magnitude of the control current changes with the modified control quantity in addition to the steering torque.
For example, if the modified control quantity is set at a large value, the control current calculated from the output of the torque sensor goes beyond the predetermined range despite correct calculation by the CPU. In turn, if the predetermined range is set to avoid such a problem, calculation problems, if any, may not be detected, and concern arises that the steering assist system cannot be deactivated in case of an anomalous calculation.
The present invention has been made to solve such problems, and an object of the present invention is to provide a control system for the electric power steering apparatus that has a CPU monitoring means for reliably detecting calculation problems in the CPU regardless of the magnitude of modified control.
According to one aspect of the present invention, a control system for an electric power steering apparatus includes a control current calculating means (means for calculating an assist current) for calculating an assist current to be provided to a motor that assists a steering and monitoring means for monitoring calculation by the control current calculating means. This control current calculating means has a basic control quantity calculating means for calculating a basic control quantity for the assist current from a steering torque of a steering wheel, and a modified control quantity calculating means for calculating a modified control quantity to correct the basic control quantity based on steering conditions of the steering wheel. The monitoring means has a first anomaly deciding means for deciding whether the calculation result of the basic control quantity calculating means is normal or anomalous in light of the steering torque of the steering wheel and a second anomaly deciding means for deciding whether the calculation result of the modified control, quantity calculating means is normal or anomalous in light of the steering conditions of the steering wheel.
According to this configuration, it becomes possible to decide whether the calculation results are normal or anomalous by monitoring the calculation result of the basic control quantity calculating means and that of the modified control quantity calculating means, separately. The first anomaly deciding means decides whether the calculation result of the basic control quantity calculating means is normal or anomalous only in light of the steering torque of the steering wheel. Meanwhile, the second anomaly deciding means decides whether the calculation result of the modified control quantity calculating means is normal or anomalous only in light of the steering conditions of the steering wheel. It becomes thereby possible to detect with reliability any anomaly in each calculating means, regardless of the magnitude of the modified control quantity.
According to a second aspect of the invention, the control system according to the first aspect has a motor driving circuit for power assist, and the motor driving circuit deactivates the motor if at least either the first anomaly deciding means or the second anomaly deciding means detects an anomaly. In other words, when it is decided that any one of the basic control quantity and the modified control quantity calculated by the respective calculating means is anomalous, the motor is deactivated. Then it becomes possible to prevent the wrong torque assist of the motor in the event of an anomalous calculation.
According to a third aspect of the invention, in the control system according to the first aspect of the invention, the second anomaly deciding means decides the calculation result of the modified control quantity calculating means as an anomaly if the modified control quantity is equal to or larger than a prescribed value, such as an upper limit. Then whether the calculation result of the modified control quantity calculating means is normal or anomalous can be decided by a simple monitoring of the upper limit, and a simplified logic circuit can be adopted in the second anomaly deciding means.
According to a fourth aspect of the invention, the control system of the first aspect further comprises a third anomaly deciding means for deciding whether the motor driver signal provided by the motor driving circuit to the motor is normal or anomalous in light of the assist current. According to this configuration, whether the motor driver signal is normal or anomalous can be decided in light of the assist current provided by the control current calculating means. Then a calculation problem that may arise during calculation of the motor driver signal based on the assist current can be reliably detected.
According to a fifth aspect of the invention, in the control system of the fourth aspect, the motor driving circuit deactivates the motor if at least one of the first anomaly deciding means, the second anomaly deciding means, or the third anomaly deciding means detects an anomaly. If one of the basic control quantities calculated by the basic control quantity calculating means, the modified control quantity calculated by the modified control quantity calculating means and the motor driver signal calculated based on the assist current is decided as anomalous, the motor is deactivated. Then motor torque assist based on an anomalous calculation can be prevented, and improved driving can be ensured.
According to a sixth aspect of the invention, in the control system of the fourth aspect, one of the first anomaly deciding means, the second anomaly deciding means, and the third anomaly deciding means is built in a custom IC. Then a monitoring section (anomaly deciding means) can be provided by an inexpensive custom IC along with the first and third anomaly deciding means to which logic circuits are applicable. All of the above means for performing an operation are also operational as specific devices.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.